String Processing

• Strings represent data that may need to be processed
• Common to deal with CSV (Comma separated value) or similar formatted data
• Standard library functions can help, but processing involves designing algorithms

More Convenience functions in C

• There is a ctype.h library that provides several "check" functions for single characters: isalpha(c), isdigit(c), islower(c), isupper(c), isspace(c), conversion: toupper(c), tolower(c), etc.

String Formatting

• In C: you can use printf to print a formatted string to the standard output
• You can use sprintf to "print" to a string instead
• You use it exactly the same as printf but it takes an additional first argument: the string you want to "print" to
• It is YOUR responsibility to ensure the string is big enough

char s[100];
int x = 10;
double pi = 3.1415
char name[] = "Chris";

//standard output:
printf("Hello, %s, you have a value of %d, and pi is %.3f\n",
name, x, pi);

sprintf(s, "Hello, %s, you have a value of %d, and pi is %.3f\n",
name, x, pi);

char *result = (char *) malloc(sizeof(char) * (strlen(s) + 1));
strcpy(result, s);
//now I have a string, result of the EXACT size I need~!

• General strategy: you can use a "temporary buffer" like above to format a string and then once formatted, you can use strlen and malloc to create a string of the exact size you need, then copy it over, and return it if needed

Java

• Java: String.format which uses a printf- style placeholders and returns a String

int x = 10;
double pi = 3.1415
String name = "Chris";

String s = String.format("Hello, %s, you have a value of %d, and pi is %.3f\n",
name, x, pi);

Java: Mutable Strings

• In Java the String class is immutable
• In general, immutability is a Very Good Thing: immutable objects are automatically thread-safe
• However, sometimes you might need a mutable version: StringBuilder: you can change the contents, it shrinks/expands automatically

StringBuilder sb = new StringBuilder();
sb.append("hello");
sb.append(", ");
sb.append("World");
sb.setCharAt(0, 'H');
String s = sb.toString();

Tokenization

• Some strings may contain formatted data: CSV, TSV (tab separated values)
• We want to "split" a formatted string up into tokens along some delimiter (tab, space, commas)
• In C: you can use strtok, it takes two arguments: the string you want to tokenize and a string containing one or more delimiters
• The first time you call it, you pass the string you want to tokenize
• Each subsequent call, you pass in NULL in order to continue tokenizing the same string
• strtok returns a pointer to the next token, NULL when there are no more tokens
• You can use multiple delimiters, but generally stick with one

char s[] = "Chris,Bourke,UNL,Omaha,NE";
//strtok returns a pointer to the next token, so we need to store that:
char *token = NULL;
//first argument is the string you want to tokenize
//second argument: the delimiter
//it returns a pointer to the "next' token
token = strtok(s, ",");
//the pointer returned, points to a part of the tokenized string
//with the delimiter *replaced* with a null termianting character
//print it:
printf("The first token is %s\n", token);

//next token: passing in NULL tells strtok to continue tokenizing
// where it left off
token = strtok(NULL, ",");`
printf("the second token is %s\n", token);

//how do we continue?
//When no further tokens are available, strtok returns NULL
while(token != NULL) {
  printf("token = %s\n", token);
  token = strtok(NULL, ",");
}

Java

• All you need to use is the .split method

String s = "Chris,Bourke,UNL,Omaha,NE";
String tokens[] = .split(",");
for(String str : tokens) {
  System.out.println(str);
}

• The split method actually supports regular expressions

String s = "Chris,Bourke,UNL,Omaha,NE";
String tokens[] = split("\\s");  //splits along any whitespace character
for(String str : tokens) {
  System.out.println(str);
}

String Comparisons in C

• In both languages, you CANNOT use the == operator to determine if two strings are the same
• Instead you must use a function or method
• In both languages, this general comparison function has the following "contract"
• It is known as a comparator function/method: given two elements, a, b it returns:
  • something negative if a < b
  • zero if a = b
  • something positive if a > b
• The relative ordering is according to the ASCII text table
• In C: int strcmp(const char *s1, const char *s2);

Java

• You can use the .compareTo

String a = "apple";
String b = "banana";
int result = a.compareTo(b); //something negative
result = b.compareTo(a); //something positive
result = b.compareTo(b); //zero

• Both languages have case-insensitive versions:
• C: strcasecmp
• Java: .compareToIgnoreCase()

String a = "apple";
String b = "ApPlE";
int result = a.compareToIgnoreCase(b); //zero